Paints and inks using water as a medium are used in a wide range of fields because they are safe and have a small impact on environment. Aqueous inks that can be applied to ink jet recording are, for example, usable in a closed space and therefore expected to replace electrophotographic products for household, office, and industrial uses.
Aqueous inks are, however, used in printing in a liquid state of which the pigment concentration is much lower than that of toner, and a printed matter therefore has a small optical density (also referred to as OD value, image density, or coloring density); in particular, in the case where a recording medium is plain paper, the ink easily soaks into the paper, which results in the problem that high optical density is less likely to be produced.
Optical density itself is a property inherent in aqueous inks. In general, aqueous inks are prepared by diluting an aqueous pigment dispersion or an aqueous pigment composition having a high pigment concentration, which is called a pigment paste, with water or an aqueous medium; hence, the optical density is considered not only in an ink having a low pigment concentration but also in a pigment dispersion itself having a high pigment concentration.
Patent Literature 1, for instance, discloses a black pigment dispersion that is used in an aqueous ink jet recording pigment ink to produce high coloring density (OD value) in printing on plain paper; in particular, such a black pigment dispersion is an aqueous black pigment dispersion that contains carbon black having a predetermined primary particle size, a specific surface area, and DBP oil absorption and a resin having a specific composition and acid value.
In this disclosure, however, the type and properties of the pigment are not specified, and a method for producing high coloring density also in use of general aqueous pigment dispersions is not explained.
One of known additives of inorganic particles is fumed silica (fumed silica particles). Fumed silica is generally used as a thickener in a UV ink for flexography with a printing plate (for example, see Patent Literature 2) or as a pigment for the receiving layer of an ink jet recording medium (for instance, see Patent Literature 3).
In a known example of using fumed silica in an ink jet recording ink, hydrophobic fumed silica that is an example of hydrophobic silica fine particles is used to address bronzing of an ink containing a pigment of a chromatic color (for instance, see Examples 1 and 2 in Patent Literature 4).
Use of hydrophilic fumed silica in an aqueous pigment dispersion for producing high optical density, however, has not been known.